In tank venting in internal combustion engines, it is known to vent the fuel vapors that form because of and as a function of certain parameters (fuel temperature, fuel quantity, vapor pressure, air pressure, flushing quantity, et cetera) not merely into the open air but instead to deliver them to the engine for reuse, preferably via an intermediate storage means filled with activated charcoal. The activated charcoal container receives the fuel vapors that form in the tank, for example when the vehicle is stopped, and typically communicates via a line with the intake region of the engine.
In this connection, it is also known to prevent or minimize a possible increase in exhaust gas emissions, due to such an increase in the quantity of fuel/air mixture resulting from tank venting, by permitting tank venting only at certain engine operating states. In this connection, reference can be made to "Motronic", C5/1, August 1981 and published German patent application DE-OS No. 28 29 958.
The intermediate storage container having the activated charcoal filter is in a position to store fuel vapors up to a certain maximum amount; flushing or regeneration of the filter during engine operation takes place as a result of the negative pressure developed by the engine in the intake area; to this end, the filter has a further opening to the ambient air. Necessarily, however, even if regeneration of the intermediate storage container is permitted only under certain operating conditions, this tank venting produces an additional quantity of fuel/air mixture; since this quantity is unmeasured, or cannot be measured at reasonable cost, the fuel metering signal (in a fuel injection system, for instance the duration of the injection control command t.sub.1) which is normally prepared with very high accuracy, at high cost for computation, is made incorrect, as is the resultant fuel quantity delivered to the engine. At certain throttle flap angles, which can be associated with a predetermined threshold value .alpha..sub.tank, the flows of fuel from the tank venting can have a quite substantial effect on the lambda value. An additional fuel quantity of this kind, which in particular also affects vehicle performance under certain conditions, and where in extreme cases the tank venting mixture may comprise virtually 100% air or 100% fuel vapor, is thus unacceptable, even if the influence of this disruptive factor is associated, for example, with the intake pipe pressure developed by the engine, or if the delivery of the tank venting mixture is precluded entirely in particularly sensitive operating states, such as idling, by providing an electronic on/off control. Furthermore, tank venting operation in general is particularly disruptive in certain internal combustion engine fuel metering systems which, in order to effect the actual determination of the quantity of fuel to be delivered to the engine at every instant, as a function of rpm and the load status, evaluate characteristic fields which in a feedback learning process (known as adaptation) are amenable to further general and structural adaptation. This adaptation is typically effected by evaluating an actual value signal, for instance deriving from a lambda sensor; the adaptation will necessarily be defective if an arbitrarily variable figure is additionally used to determine the actual value.